In these days, the importance of the development for fuel cells, which use hydrogen for power generation, is on the increase. Use of fuel cell can not only replace the depleting fossil fuels but also prevents environmental woes such as global warming caused by excessive generation of carbon dioxide. Fuel cells also have various advantages to satisfy the environmental and technical demands. Fuel cells directly convert a chemical energy into an electric energy using hydrogen as a fuel. Because energy efficiency is high and only water is the reaction product as an exhaust material, fuel cell technology is an eco-friendly energy technology.
The operating mechanism of a fuel cell starts from production of electrons and hydrogen ions (protons) by oxidizing a fuel such as hydrogen, natural gas, and methanol on the anode side. Protons generated on the anode side move to the cathode side through an electrolyte membrane, and electrons generated on the anode side are supplied to an external circuit through a separator. The protons combine with oxygen supplied to the inside of the fuel cell as oxidant and produce water.
Fuel cells may be classified into several categories, a polymer electrolyte membrane fuel cell (PEMFC), a phosphoric acid fuel cell (PAFC), a molten carbonate fuel cell (MCFC), and a solid oxide fuel cell (SOFC), according to the type of electrolyte used. Operating temperature and materials for the components vary with the type of fuel cell.
A polymer electrolyte (proton exchange) membrane fuel cell may operate at a relatively low temperature of about 80-120° C. and may have a high power density and thus may be used as a power source fix automobiles and homes. Such a PEMFC includes a stack having multiple unit cells separated by a plate called separator. Each unit cell comprises electrolyte as a polymer ion exchange membrane which is inserted between the two catalyst electrodes. The electromotive fierce of one unit cell is only several hundreds mV (in the case of a unit cell conventionally used, about 700 mV) and, when a fuel cell is actually used in a device, hundreds of serial unit cells need to be stacked. Due to their advantages ci fuel cells, many researches and developments have been made over the last decade, and prototype fuel cells have been produced. However, previously produced fuel cells are very expensive and wide use of them in industry is limited. For the general application of fuel cells in industries, the manufacturing cost should be reduced through the component development and component price reduction. In a stack of a fuel cell, separator is an important component and takes a weight ratio of about 80% and a price ratio of about 40% of the stack. Development of a low-priced and light weighted separator is essential for the wide application of fuel cells in industries and private homes.
A separator serves as a blocking plate between the unit cells, contacts a membrane electrode assembly (MEA), and transmits electricity generated in a unit cell. Since the inside of a unit cell is in a corrosive environment, the separator has to have good corrosion resistance and the resistivity of the surface of the separator that contacts the MEA should be low for good electric efficiency. In addition, since the number of separators used in one fuel cell stack is several hundreds, it is preferable to fabricate the separator as thin as possible to make the entire stack compact.
At present, graphite is generally used as the separator material for fuel cell, and hydrogen and air paths are formed in the separator by molding or milling. The graphite separator has advantages of good electrical conductivity and good corrosion resistance. However, due to the high cost of the graphite material itself and high processing cost of a plate, the price of the separator is very high. In addition, since the graphite separator is fragile, it is not easy to process the graphite separator to a thickness of 4-5 mm or below. Thus, it is difficult to reduce the size of a fuel cell stack consisting of several tens to several hundreds of unit cells to a desirable dimension.
As an alternative for graphite separator materials, many attempts were made to use metals for separator materials. Metals have many properties required for the separator and have advantages of low material and processing costs. However, metals can corrode in oxidative environment inside the stack and oxide films that have large electrical resistivity can easily form on the surface of the metallic separator. Problems such as membrane poisoning or an increase of contact resistance can occur. Corrosion of metallic separator can cause the formation of defects in the metallic separator and the poisoning of catalyst and electrolyte by the diffusion of metallic ions into the electrolyte membrane. When the catalyst is poisoned, the activity of the catalyst is degraded, and, when the electrolyte is poisoned, the ion conductivity of the electrolyte is degraded. The performance of the fuel cell will also be degraded. In addition, because the corroded metal is in contact with the MEA, interfacial contact resistance (ICR) increases and the performance of the fuel cell will be further aggravated.
Such a corrosion problem is the largest obstacle for the application of the metallic separator. In order to improve the corrosion resistance of the separator, surface treatment of high chromium alloys and precious metals has been attempted. In general, surface treatment often performed to improve the surface characteristics of materials. Surface treatment includes the forming a film or making a chemically stable compound on to the surface of base metals.
When a chromium nitride layer is coated on the surface of the high chromium alloy steels, the corrosion characteristics of the material can be improved. However, the cost of high chromium alloy steel can be very high and the corrosion resistance of the coated layer is not sufficient for fuel cell. In case of forming chromium nitride layers on chromium steels, two types of compounds can form and their physical properties are quite different. CrN, Cr2N, or both of the nitrides can form depending on the nitriding condition, such as temperature and nitrogen partial pressure. Chromium nitride layer formed on the surface of a high chromium alloy is demonstrated in FIG. 1. Both types of the chromium nitrides were formed. The electrical resistivity of CrN compound is very high, 10 times higher than that of Cr2N compound. It is very much favorable only to form a chromium nitride, Cr2N, which has better electrical properties, on the metallic separator.